The horizontal deflection circuit output transformer of a television receiver is known to generate inter alia by means of a high voltage coil and a high voltage rectifier, a high voltage of the order of magnitude of 25 KV for the picture tube. This voltage is obtained by rectifying the pulse-shaped flyback, or retrace, voltage produced in the transformer. Due to this high pulse-shaped voltage, the output transformer must meet especially high demands regarding its voltage stability since such high pulse voltages could easily lead to sparkovers and arc discharges.
Generally such a transformer includes a frame-shaped core having an air gap and, if it is designed as a singlearm transformer, the primary winding and the high voltage winding are arranged one on top of the other on the same arm of the core. For reasons of cost and weight, it is desirable to give the core the smallest possible dimensions. As a result, the high voltage coil takes up practically the entire length of an arm of the core, i.e., it extends up to the adjacent arms of the core which depart from this arm at a right angle. Particularly at a point where the end of the high voltage coil lies closely against an adjacent core arm, there also exists the high pulse voltage of the high voltage coil. For that reason, this point poses particular difficulties in realizing the required voltage stability.
Testing of such transformers is performed with a voltage exceeding rated voltage by 50%, in which case there appear even higher pulse voltages which reach an order of magnitude of 40 KV across the entire high voltage coil.